The term “integrated circuits” in this document refers to dies, usually consisting substantially of a semiconductor material, and having electrical contacts formed on their upper surfaces. During a packaging process, the dies are embedded in a resin body from which electrically conductive leads extend.
A known packaging method is illustrated in FIG. 1, which consists of FIGS. 1a and 1b. FIG. 1a is a top view of a central portion 1 of a leadframe, while FIG. 1b is a cross-sectional view along the line A-A. The central portion 1 of the leadframe is made of a conductive material. For example, it may be a copper plate. The leadframe includes further portions (not shown) outside the region 1, including leads. An integrated circuit 3 is adhered on a “die pad” area within the portion 1. The integrated circuit 3 includes electrical contacts on its upper surface. Certain of these electrical contacts of the chip are bonded by wire bonds (not shown) to respective contacts on the leads of the leadframe. However, certain of the contacts 5 of the integrated circuit 3 are to be connected to ground. To do this, the central portion 1 of the leadframe is connected to ground (not shown), and wire bonds 7 are formed between contacts 5 and respective contacts on the copper 1. In general, it is found that the adhesion of the wire bonds 7 with the copper 1 is not adequate, and therefore silver regions 9 are plated in the locations of the copper 1 where the wire bonds 7 terminate. Subsequently, the integrated circuit 3, wire bonds 7 and silver plated regions 9 are encased in resin. During the same resin molding process, the wire bonds (not shown) between the integrated circuit 3 and the leads (not shown) are encased in a resin body.
This process suffers from a number of problems. A first is that, in subsequent use of the device, the resin body may delaminate from the portion 1 of the leadframe. This delamination propagates rapidly across the surface of the portion 1, and can cause the wires 7 to be torn from the silver plating 9.
A second problem is that, when the integrated circuit 3 is adhered to the portion 1 of the leadframe, the adhesive may leak from beneath the integrated circuit 3 and cover some of the regions of the silver plating 9. In this case, it may be impossible to form reliable wire bonds 7 to the regions of the silver plating 9.
A third problem is that, since the regions of silver plating 9 are arranged to coincide with locations of the contacts 5 on the integrated circuit 3, the silver plated leadframe must be designed using knowledge of the integrated circuit 3. In other words, different designs of leadframes are required for packaging different integrated circuits. This reduces the flexibility of the packaging process. This problem is addressed by a second known packaging process shown in FIGS. 2a and 2b. Elements in these figures that correspond to those of FIG. 1 are shown by the same reference numerals. The process differs from that of FIG. 1 only in that regions 9 are replaced by a ring 11 of silver plating along the external periphery of the portion 1 of the leadframe. The wire bonds 7 of FIG. 1 are replaced-by wire bonds 13 between the contacts 5 and the ring 11. Thus, a wide variety of different types of integrated circuit 3 can be packaged with the same type of leadframe. Unfortunately, this arrangement suffers from the problem that the ring 11 must be along the external periphery of the portion 1 of the leadframe in order for the die pad area to be able to accommodate integrated circuits of varying size. So, when the integrated circuit 3 is relatively small, the wires 13 must be much longer than the wires 7 in the arrangement of FIG. 1. Furthermore, the resin's adhesion to silver tends to be less than adhesion to copper, so the arrangement of FIG. 2, in which a larger surface of the portion 1 of the leadframe is covered by silver plating 11, suffers from a higher risk of delamination between the resin and the leadframe.